1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly, to a color filter on thin film transistor type liquid crystal display device having a black matrix in an outside region immediately adjacent to a pixel region.
2. Discussion of the Related Art
A liquid crystal display (LCD) device is a display device for displaying an image using liquid crystal molecules having a refractive anisotropy and a dielectric anisotropy. In general, an LCD device includes an array substrate in which unit pixels are arranged in a matrix form, a color filter substrate facing the array substrate and displaying an image in color, and a liquid crystal layer between the array substrate and the color filter substrate.
A TN (twisted nematic) mode LCD device which provides vivid colors with high brightness is being widely used these days. The TN mode LCD device includes an array substrate in which unit pixels are arranged in a matrix form, a color filter substrate having a color filter layer and facing the array substrate, and a liquid crystal layer formed such that liquid crystal molecules are twisted between the two substrates. In addition, a thin film transistor (TFT), which is a switching device for a unit pixel, is formed at each pixel of the array substrate. Thus, the array substrate is often referred to as a TFT array substrate.
Because the LCD device cannot generate light by itself, the LCD device includes a backlight assembly for supplying light from the outside. In general, in a TN mode LCD device, light advances from under the TFT array substrate and passes through the liquid crystal layer and the upper color filter layer to display an image. Part of the light advancing from under the TFT array substrate is unnecessary, which should be blocked. Therefore, a black matrix is formed to block the unnecessary light.
In general, the black matrix is formed of an opaque metal thin film or an organic film on a color filter layer. However, the LCD device in which the black matrix is formed on the color filter substrate is disadvantageous in that it is difficult to precisely attach the array substrate and color filter substrate together. Therefore, a color filter on TFT (COT) type LCD device has been proposed.
In a COT type LCD device, a black matrix and a color filter layer are formed on an array substrate so that an LCD panel forming process concentrates on the array substrate, and the alignment between the array substrate and the color filter substrate during an attachment is easily facilitated.
FIG. 1 is a cross-sectional view illustrating a COT type LCD device in accordance with the related art. In FIG. 1, a COT type LCD device includes a color filter layer 110 and a black matrix 111 on a lower substrate 101. The COT type LCD device also includes an upper substrate 150 facing the lower substrate 101 and a liquid crystal layer 140 between the lower and upper substrates 101 and 150. The lower substrate 101 includes gate lines (not shown) and data lines 108 intersecting each other and defining a plurality of unit pixels. In addition, the lower substrate 101 includes a TFT at one side of every unit pixel, a color filter layer 110 at every unit pixel, and a black matrix 111 between the color filter layers 110.
Although not shown, the plurality of TFTs are arranged in a matrix form on the lower substrate 101, which is a transparent glass substrate. The TFT includes a gate electrode 103 for receiving a scan signal, a gate insulation layer 102 formed on the gate electrode 103, an active layer 104, which is a semiconductor layer formed on the gate insulation layer 102, and an ohmic contact layer 107 contacting the active layer 104 with source and drain electrodes 105 and 106. Further, a passivation layer 109 for protecting and insulating the TFT from the outside is formed on the TFT.
The black matrix 111 is formed on the passivation layer 109. The black matrix 111 is arranged in a matrix form for blocking the unnecessary light leaked from and passing through a reverse tilt domain region around the gate line (not shown) and the data line 108 and the like. In addition, the color filter 110 having one of red, green and blue sub-color filters is formed on the passivation layer 109 at each of the unit pixels.
A pixel electrode 112 also is formed on the passivation layer 109. In particular, a contact hole is formed in the color filter 110 and the passivation layer 109 for exposing the drain electrode 106 of the TFT, such that the pixel electrode 112 is connected to the drain electrode 106 through the contact hole.
The upper substrate 150, which is a glass substrate, includes a common electrode 151 and an alignment layer 152. The alignment layer 152 is formed on the common electrode 151 for aligning liquid crystal molecules of the liquid crystal layer 140.
The lower substrate 101 generally is referred to as a TFT array substrate. The TFT array substrate includes a pixel region where the pixel electrode 112 is formed, pad regions where a gate pad and a data pad for supplying a gate voltage and a data voltage are formed, a seal forming region where a sealant for attaching the upper substrate and the array substrate together is formed, and an outside region between the pixel region and the seal forming region.
FIG. 2 is a cross-sectional view illustrating edge portions of the COT type LCD device shown in FIG. 1. As shown in FIG. 2, the array substrate includes a pixel region A, an outside region O adjacent to the pixel region A, a seal forming region P where a seal line 170 is formed, and a pad region Q in which various pads are formed. A spacer 172 is formed between an array substrate 160 including a lower substrate 101 and an upper substrate 150. A gate pad electrode 176 is formed on the lower substrate 101.
An electrostatic discharge (ESD) circuit 171 is formed between the pixel region A and the seal line 170, namely, at the outside region O. The ESD circuit 171 may be constructed by a combination of a plurality of TFTs, and one electrode thereof is connected to the pad region Q and its other electrode is connected to other lines of the pixel regions by Indium Tin Oxide (ITO). Therefore, when the external static electricity enters through the pad region Q, the ESD circuit 171 is operated to disperse the static electricity to the LCD panel and to block the static electricity from being applied to the pixel region A, thereby protecting the pixel.
However, light leakage of a backlight occurs at the outside region O because the outside region O having the ESD circuit 171 is covered with a transparent passivation layer. In order to solve such a problem, there has been an attempt to prevent the leakage of light using a top case 175 encompassing the LCD panel. However, because the top case 175 should not contact with a polarization plate 174 installed at an outside of the upper substrate 150, the top case 175 does not completely cover the outside region O, thereby failing to completely prevent light leakage.
Thus, an additional black matrix 173 of an opaque metal layer is formed on the upper substrate 150 corresponding to the outside region O. As a result, a black matrix forming process is performed twice, one in an array substrate fabricating process and another in an upper substrate fabricating process, thereby reducing production efficiency and increasing fabrication costs.